Thermoplastic TPO is diverse in variety, and even the same variety has different usage and processing characteristics due to different resin molecule and additive ratios. In addition, in order to change the characteristics of the original variety, various chemical methods such as copolymerization and cross-linking are often used to introduce a certain percentage of other monomers or polymers into the original resin structure, so as to change the structure of the original resin and become a modified product with new improved physical and processing properties.
For example, ABS is a modified copolymer with acrylonitrile and butadiene introduced into the polystyrene molecule, which can be regarded as modified polystyrene and has excellent comprehensive performance and process characteristics.
Due to the wide variety and complex performance of TPO thermoplastic, even the same type of plastic has different applications for injection molding and extrusion. Therefore, this chapter mainly introduces various thermoplastic TPO used for injection molding.
As mentioned earlier, the form and calculation of shrinkage of thermoplastic TPO, the factors affecting the shrinkage of thermoplastic TPO are as follows:
Plastic varieties. During the molding process of thermoplastic TPO, there is still volume change caused by crystallization, strong internal stress, large residual stress frozen in the plastic parts, and strong molecular orientation, so the shrinkage rate is larger than that of thermosetting plastics, with a wide range and obvious directionality. In addition, the shrinkage after molding, annealing or moistening treatment is generally greater than that of thermosetting plastics.
Part characteristics. During molding, the molten material contacts the surface of the cavity to form a low-density solid shell immediately on the outside. Due to the poor thermal conductivity of the plastic, the inner layer of the plastic part cools slowly to form a high-density solid layer with large shrinkage. Therefore, the wall thickness, slow cooling and thick high-density layer have large shrinkage. In addition, the presence or absence of inserts and the layout and quantity of inserts directly affect the flow direction, density distribution and shrinkage resistance, so the part characteristics have a great influence on the shrinkage size and directionality.
Factors such as the form, size and distribution of the feed port directly affect the flow direction, density distribution, pressure holding and shrinking compensation effect, and the molding time of TPO thermoplastic. The direct feeding port and large cross-section of the feeding port (especially the thicker one) have small shrinkage but large directionality, while the wide feeding port and short length have small directionality. Those close to the feeding port or parallel to the flow direction have large shrinkage.
Molding conditions. High mold temperature, slow cooling of molten materials, high density and large shrinkage, especially for crystalline materials, which have high crystallinity and large volume changes, so the shrinkage is larger. The temperature distribution of the mold and the uniformity of cooling and density inside and outside the plastic parts are also related, which directly affect the size and directionality of the shrinkage in each part. In addition, the holding pressure and time also have a great influence on the shrinkage. The higher the pressure and the longer the time, the smaller the shrinkage but the larger the directionality.
High injection pressure, small viscosity difference of molten material, small interlayer shear stress, and large elastic rebound after demolding can also moderately reduce the shrinkage, while high material temperature and large shrinkage, but small directionality. Therefore, when TPO thermoplastic are molded, adjusting mold temperature, pressure, injection speed, and cooling time can also appropriately change the shrinkage of plastic parts.
When designing the mold, based on the shrinkage range of various plastics, the wall thickness and shape of the plastic parts, feed port form, size and distribution, the shrinkage rate of each part of the plastic parts is determined by experience, and then the cavity size is calculated.